When a slippage restraining control for restraining slippage of a wheel, for example, an anti-skid control (i.e., anti-lock braking system control, or ABS control) or a traction control (i.e., TCS control), or the like, is executed while a vehicle is traveling on a road surface having different coefficients of function at wheels on the right and left sides (i.e., hereinafter referred to as a μ split road) (i.e., the slippage restraining control referred to as a μ split control hereinafter), differences (i.e., braking force difference when the ABS control is applied; driving force difference when the TCS control is applied) are generated between longitudinal forces (i.e., friction force in an acceleration or deceleration direction generated between a road surface and a tire; also referred to as driving/braking force) at wheels on the left side and right side. A yaw moment which deviates a trajectory of a vehicle based on the differences in longitudinal forces (i.e., hereinafter referred to as a longitudinal force difference-based yaw moment) is generated.
In order to restrain the deviation of the trajectory of the vehicle by the longitudinal force difference-based yaw movement, it is required to reduce (i.e., to cancel) the longitudinal force difference-based yaw moment by correcting steering angle of wheels in a reverse direction to the deviated direction of the vehicle by operating a steering wheel in a reverse direction to the deviated direction of the vehicle. The foregoing operation to correct the steering angle of the wheels in a reverse direction to the deviated direction of the vehicle is referred to as a counter steering operation. The counter steering operation requires that a driver be highly skilled.
In light of the foregoing, according to a known apparatus described in JP2540742B, pressure difference of hydraulic brake pressure of wheels at the right side and left side when braking is applied by an anti-skid control system, calculating corrected steering angle of rear wheels or front wheels of the vehicle in response to the pressure difference, and steering the rear wheels or the front wheels in response to the corrected steering angle so that braking is applied safely without changing a course of the vehicle even when a sudden braking operation is applied on a μ split road.
Notwithstanding, with a system which controls steering angles of the both front wheels and rear wheels of the vehicle on the basis of difference of the longitudinal forces between wheels on the right and on the left, it is required to determine steering angles considering a percentage of distribution of a stabilization moment to (i.e., a contribution rate of) front wheels and rear wheels in order to restrain the longitudinal force difference-based yaw moment.
A need thus exists for a steering angle control apparatus for a vehicle, which executes steering angle control of front wheels and rear wheels which restrains deviation of a vehicle by a yaw moment based on longitudinal force difference of wheels on the right side and on the left side on a μ split road (i.e., referred to as correcting steering angle control) and which determines a contribution rate of the front wheels and the rear wheels for the correcting steering angle control to restrain deviation of the vehicle on the μ split road.